Microheat exchangers are made of thermally conductive material and are used to transfer heat from a heat generating device, such as an integrated circuit or a laser diode, to a fluid flowing through fluid pathways within the microheat exchanger. Microheat exchangers are commonly made of metal, such as copper, and electrical isolation is often required between the heat generating device and the microheat exchanger. Some ceramic materials are thermally conductive yet electrically resistant. For this reason, such ceramic materials are often used as an intermediate material between a heat generating device and a microheat exchanger to provide electrical isolation while still maintaining thermal conductivity. However, it is not practical to connect a heat generating device directly to ceramic. Instead, the heat generating device is coupled to a conductive pad, typically made of a conductive metal such as copper. In such a configuration, the ceramic is middle layer between the conductive copper pad coupled to the heat generating device and the microheat exchanger.
In order to provide efficient heat transfer from the heat generating device to the microheat exchanger, a good thermal interface between ceramic and copper is necessary. A direct bonded copper (DBC) method uses a high temperature joining process to bond a copper sheet to a ceramic plate in the presence of a protective gas atmosphere having small amounts of oxygen (50-200 ppm). Exemplary DBC methods are described in U.S. Pat. No. 6,297,469 and U.S. Pat. No. 7,036,711, which are hereby incorporated in their entirety by reference. Three commonly used ceramic materials are beryllium oxide (BeO), aluminum oxide (Al2O3), and aluminum nitride (AlN). Oxygen and copper bond together under high temperature. The copper and ceramic are heated to a carefully controlled temperature, in an atmosphere of nitrogen and a small percentage of oxygen. The temperature used is in the range between 1950 and 1981 degrees Fahrenheit, which is close to the melting temperature of copper. Under these conditions, a copper-oxygen eutectic forms which bonds successfully both to copper and the ceramic, thereby bonding a copper layer to a ceramic layer. The copper layer is used as a conductive pad to be coupled to a heat generating device. The ceramic layer is typically soldered to the top of the microheat exchanger.
Many problems exist with bonding in general and the DBC technique in particular. First, application of high temperature to rigid ceramic plates often results in cracking of the ceramic. Second, microvoids are formed at the interface of the bonded copper and ceramic layers. The microvoids are due to the imperfections and irregularities in the contact surfaces of the copper and ceramic layers. For applications where a large heat generating device, or multiple heat generating devices are coupled to a single ceramic plate, the size of the ceramic plate is larger. However, the larger the ceramic plate, the greater the impact of the microvoids. Presence of microvoids reduces thermal efficiency. Further, presence of microvoids increases the chances that the copper layer and the ceramic layer will delaminate because there is not a perfect bond across the entire interface surface.
Third, the thermal coefficient of expansion for copper is much greater than that for ceramic. During the high temperature DBC process, the copper layer expands more so than the ceramic, at which point the ceramic layer and the copper layer are bonded. However, upon cooling the copper layer contracts more so than the ceramic, due to the differing thermal coefficients of expansion, which leads to warping and possible cracking of the bonded copper-ceramic assembly.